A conventional photon detector using a communication wavelength band avalanche photo diode (hereinafter, avalanche photo diode is called “APD”) applies a rectangular wave pulse at an incident timing of a photon in a state in which a DC bias slightly lower than a breakdown voltage is applied to the APD, and detects the photon by observing its response waveform (for example, refer to Patent Document 1).
In this detecting system, because the APD functions similarly to a capacitor at the time of detecting no photon, the response waveform from the APD becomes a differentiated waveform of the rectangular wave, and constantly has a positive charge pulse and a negative discharge pulse. At the time of detecting the photon, because an avalanche current flows, a signal pulse is superimposed on the response waveform. In general, the signal pulse that has become larger than the charge pulse is determined by threshold discrimination to enable photon detection.
For that reason, there arises such a problem that the photon detection is not enabled unless the signal pulse becomes larger than the charge pulse, and available APDs have been remarkably limited. On the other hand, there has been also proposed a detecting system in which two APDs are used and the respective response signals are multiplexed in opposite phases to each other to offset the charge pulses (for example, refer to Patent Document 2).
In this detecting system, it is difficult to perfectly offset the mutual charge pulses, and hence there has been proposed a method in which a sinusoidal wave is input to the APD instead of the rectangular wave pulse to eliminate the occurrence of the charge pulse and the discharge pulse. In this case, the response waveform of the APD at the time of detecting no photon is the sinusoidal wave, and at the time of detecting the photon, a signal of a non-sinusoidal wave component is superimposed on the response waveform.
For that reason, there has been proposed a system in which the sinusoidal wave component is removed with the use of a band removing filter to extract a signal component (for example, refer to Non-patent Document 1). Problems with this detecting system reside in that it is difficult to obtain the band removing filter excellent in the characteristic with which the sinusoidal wave components are removed as much as possible and a drive frequency cannot be freely changed because of the frequency dependency of the filter.
Patent Document 1: JP 2005-114712 A
Patent Document 2: JP 2003-243691 A
Non-patent Document 1: N. Namekata, S. Sasamori, and S. Inoue, “800 MHz Single-photon detection at 1550-nm using an InGaAs/InP avalanche photodiode operated with a sine wave gating,” Optical Express vol. 14, No. 21, 10043, 2006.